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Heavy duty trucks present some significant air-quality and pollution problems. Their diesel engine exhaust contains a mixture of gases and solid particles and the solid particles, known as diesel particulate matter or diesel PM, have been linked to a variety of health and breathing problems, particularly in densely populated areas. Diesel engines also account for a significant portion of worldwide greenhouse gas emissions (primarily CO2), adding to problems associated with global climate change. Some progress has been made to trap the particulate matter produced by diesel engines to improve air quality, but little has been done so far to reduce their climate-change-inducing exhaust emissions.

Moving Away From Diesels

“We’ve been working for a number of years on ways to make engines for cars and trucks cleaner and more efficient, and we’ve been particularly interested in what you can do with spark ignition [as opposed to the compression ignition used in diesels], because it’s intrinsically much cleaner,” said Massachusetts Institute of Technology (MIT) Energy Initiative and Plasma Fusion and Science Center research scientist Daniel Cohn. Chon has been researching a variety of fuels and the use of hybrid spark ignition-electric technology in heavy trucks—his research is presented in an MIT news release. In the MIT release it is noted that, “Compared to a diesel engine vehicle, a gasoline-powered vehicle produces only a tenth as much nitrogen oxide (NOx) pollution, a major component of air pollution.”

A flex-fuel engine configuration for a heavy truck that allows it to run on gasoline, ethanol, methanol, or blends of these, would have the potential to emit far less greenhouse gas than pure gasoline engines do, according to the news release. Cohn notes that the incremental cost for the fuel flexibility is very small. If run on pure methanol or ethanol derived from renewable sources such as agricultural waste or municipal trash, the net greenhouse gas emissions could even be zero. “It’s a way of making use of a low-greenhouse-gas fuel” when it’s available, “but always having the option of running it with gasoline” to ensure maximum flexibility, Cohn says.

Tesla has a battery-electric heavy truck under development, however MIT researchers indicate that an interim step using an alcohol-fueled electric hybrid drivetrain may be important when moving away from diesel engines and to full battery electrics. (Image source: Tesla)

A Gradual Transition to Batteries

While the ultimate goal would be to power trucks entirely with batteries, the researchers say, this flex-fuel hybrid option could provide a way for such trucks to gain early entry into the marketplace by overcoming concerns about limited range, cost, or the need for excessive battery weight to achieve longer range. Cohn said, “We think that’s going to be very challenging, because of the cost and weight of the batteries.” Cohn and his researchers estimate to reach the range of a current diesel engine truck would require somewhere between 10 and 15 tons of batteries “That’s a significant fraction of the payload” such a truck could otherwise carry, Cohn said. “Batteries are great, but let’s be realistic about what they can provide,” he said.

“We think that the way to enable the use of electricity in these vehicles is with a plug-in hybrid,” Cohn said. The engine has been under development for years. It is a highly efficient, flexible-fuel gasoline engine that would, “weigh far less, be more fuel-efficient, and produce a tenth as much air pollution as the best of today’s diesel-powered vehicles,” according to the MIT release. The combination of a hybrid drive and flex-fuel engine is “a way to enable the introduction of electric drive into the heavy truck sector, by making it possible to meet range and cost requirements, and doing it in a way that’s clean,” Cohn said.

Cohn and MIT research engineer Leslie Bromberg, “did a detailed analysis of both the engineering and the economics of what would be needed to develop such an engine to meet the needs of existing truck operators. In order to match the efficiency of diesels, a mix of alcohol with the gasoline, or even pure alcohol, can be used, and this can be processed using renewable energy sources, they found.” The MIT release also noted that, “Detailed computer modeling of a whole range of desired engine characteristics, combined with screening of the results using an artificial intelligence system, yielded clear indications of the most promising pathways and showed that such substitutions are indeed practically and financially feasible.”

The relative cost of diesel fuel has gone up and the cost advantages of diesels for heavy trucking no longer prevail. “Over time, gas engines have become more and more efficient, and they have an inherent advantage in producing less air pollution,” said Bromberg.

“We think there’s a significant rationale for trucking companies to go to gasoline or flexible fuel,” Cohn says. “The engines are cheaper, exhaust treatment systems are cheaper, and it’s a way to ensure that they can meet the expected regulations. And combining that with electric propulsion in a hybrid system, given an ever-cleaner electric grid, can further reduce emissions and pollution from the trucking sector.”

The combination of flex-fuel and hybrid electrification allows them to address two major challenges at once. “We don’t know which is going to be stronger, the desire to reduce greenhouse gases, or the desire to reduce air pollution. In the U.S., climate change may be the bigger push, while in India and China air pollution may be more urgent, but this technology has value for both challenges,” said Cohn.

Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.

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